1. Field of the Invention
The invention relates to a high-speed transmission circuit board connection structure for electrically connecting a high-speed transmission circuit board and another high-speed transmission circuit board.
2. Description of the Related Art
A high-speed transmission circuit board 40, such as shown in FIG. 4A, is used as a circuit board for transmitting a high-speed signal (high-frequency signal). The high-speed transmission circuit board 40 is composed of a laminated substrate 41 in which a first signal transmission wiring 42 and a ground wiring (not shown) are formed on a surface thereof and a ground plane 43 is formed inside (inner layer).
FIG. 4A shows a conventional structure for connecting the high-speed transmission circuit board 40 to another high-speed transmission circuit board. The other high-speed transmission circuit board in FIG. 4A is a portion where a second signal transmission wiring 46 is formed on a surface of a ceramic substrate 48. The high-speed transmission circuit board 40 and the ceramic substrate 48 are each fixed to a board connecting member 49. The board connecting member 49 is formed of a conductive member such as metal (e.g., kovar). The first signal transmission wiring 42 of the high-speed transmission circuit board 40 is electrically connected to the second signal transmission wiring 46 formed on a surface of the ceramic substrate 48 by a bonding wire 50. In addition, the ground plane 43 of the high-speed transmission circuit board 40 is electrically connected to the board connecting member 49 by a through hole 51 which penetrates the high-speed transmission circuit board 40.
Hereinafter, a wiring path composed of the ground plane 43 of the high-speed transmission circuit board 40 and the board connecting member 49 is referred to as a ground wiring, and a wiring path composed of the first signal transmission wiring 42 and the second signal transmission wiring 46 is referred to as a signal wiring.
However, when the ground plane 43 of the high-speed transmission circuit board 40 is connected to the board connecting member 49 by the through hole 51, the ground wiring increases in length. Furthermore, a distance between the signal wiring and the ground wiring is increased between the portion for electrically connecting the ground plane 43 to the board connecting member 49 via the through hole 51 and the bonding wire 50. Therefore, there is a problem that it is difficult to match impedance which depends on the distance between the signal wiring and the ground wiring.
In order to maintain the distance between the signal wiring and the ground wiring as constant as possible, the through hole 51 should be formed in the vicinity of the end portion of the high-speed transmission circuit board 40 which faces the ceramic substrate 48. However, as shown in FIG. 5, when the first signal transmission wirings 42 are densely formed in the vicinity of the end portion of the high-speed transmission circuit board 40, it is not possible to obtain the location for forming the through hole 51, hence, it is not possible to form the through hole 51 in the vicinity of the end portion of the high-speed transmission circuit board 40.
On the other hand, as shown in FIG. 4B, a high-speed transmission circuit board 53 as a build-up substrate in which a through hole 52 for conducting from a lower surface of the laminated substrate 41 to the ground plane 43 is formed inside the laminated substrate 41 may be used in order to bring the formation position of the through hole close to the vicinity of the end portion of the high-speed transmission circuit board 40. However, there is a problem that the cost of the build-up substrate is higher than the laminated substrate having a through hole formed so as to penetrate front and back surfaces.